Chiari malformation Chiari type II malformation 2021-11-10T10:35:36Z Arnold-Chiari type II malformation While Chiari I malformation is thought to result from a small posterior fossa, Chiari II occurs due to in utero malformation of the spine and cranial structures. The underlying cause of Chiari II malformation is thought to be an in utero CSF leak due to open spinal dysraphism, resulting in a small posterior fossa with distortion of the cerebellum and a diminutive cisterna magna, sometimes accompanied by ventriculomegaly and an altered shape of the calvarium. Chiari II malformation is always associated with spinal myelomeningocele or myelocele. Classical signs described on prenatal ultrasound include lemon sign and banana cerebellum sign. A type of Chiari malformation that consists of brainstem herniation and a towering cerebellum in addition to the herniated cerebellar tonsils and vermis due to an open distal spinal dysraphism/myelomeningocele.